The present invention relates to a class of catalyst solvents and their use to transform sp2 bonded C3N4, such as graphitic or amorphous carbon nitride, into sp3 bonded C3N4, such as cubic or spinel C3N4, when processed at temperature and pressure conditions lying within the thermodynamically stable, sp3 bonding region for C3N4. The present invention also relates to products made with this class of catalyst solvents, such as a polycrystalline compact of sp3 bonded C3N4 grains formed with sp3 bonding between said crystals.